In order to meet some emission regulations, selective catalytic reduction systems using externally added reducing agents may be used. In such a system, regulated emissions, such as certain nitrogen oxides, or NOx, can be reduced in a oxygen-rich environment to nitrogen and water over a catalyst when a reducing agent, such as ammonia, is added. In addition to controlling nitrogen oxide emissions, the amount of excess ammonia, or ammonia slip, must be managed. Ammonia slip is experienced when ammonia in excess of that used to reduce the nitrogen oxides passes through the catalyst unaffected and exits the catalyst (as ammonia slip).
One method for regulating ammonia slip is to use an ammonia sensor located downstream of the catalyst. The detected ammonia concentration is compared with a fixed upper threshold value. This comparison generates a correction signal that is used to control the metering of ammonia upstream of the catalyst. Allegedly, by regulating actual ammonia slip to the upper threshold value, a certain nitrogen oxide reduction is obtained. Such a system is disclosed in U.S. Pat. No. 5,369,956.
The inventors herein have recognized a disadvantage with the above system. The above system regulates to a fixed concentration value for the upper threshold ammonia slip. However, this system does not consider NOx conversion efficiency or percentage slip. While NH.sub.3 slip expressed as concentration (ppm) and as a percent are related, there is an important distinction in their use for reductant control strategy. In general, as maximum NOx conversion is approached with increasing ammonia addition (i.e., increasing NH.sub.3 /NOx mole ratio), ammonia starts to slip. After maximum NOx conversion is attained, ammonia slip increases more rapidly with increasing NH.sub.3 /NOx. For example, if ammonia slip is regulated to a constant concentration value, an ammonia setting high enough for sufficient NOx conversion at high NOx feed gas levels is likely excessive for low NOx feed gas levels, thereby wasting ammonia. Conversely, a setting at minimum detectable ammonia concentration is likely insufficient to provide high NOx conversion at high NOx feed gas levels. Further, intermediate settings may still be insufficient to provide high enough NOx conversion at high NOx feed gas levels. Thus, prior approaches can not achieve high NOx conversion with minimal ammonia slip, particularly for vehicle engines where NOx concentration levels varies widely and quickly.
In other words, because a catalyst experiences widely varying levels of engine NOx, controlling to an ammonia slip concentration results in widely varying, and less than optimum, NOx conversion efficiency.